While condition monitoring technologies can track many signals, its purpose boils down to detecting wear and preventing the eventual failure of monitored components. Not all megawatt-class wind turbine drivetrains are monitored with the intent to catch degrading components. A few frequently encountered conditions are not caused by wear, making them correctable.
These conditions, when left uncorrected, manifest themselves in damaged components along with associated collateral damage. This is the typical pay-a-little-now-or-a-lot-later scenario. The task is certainly one for condition monitoring, but this topic could be called “correctible detection”. Here’s an overview of three common correctible conditions along with how they can be detected and corrected.
It’s common between the gearbox output (or high-speed shaft) and the generator input shaft. Causes include a flexible bedplate, large temperature variations, cantilevered mounting, thermal growth, and others. It is prevalent enough that some manufacturers recommend shaft alignment at prescribed intervals while others specify laser-based tools for aligning this portion of the turbine. “Because of the dynamic movement and flexibility of the turbine, alignment tolerances are much broader than what we see in standard industrial applications,” says alignment expert Paul Berberian of Alignment Supplies Inc. “Sometimes acceptable wind-turbine-alignment specifications are 2 to 4 times higher than those found elsewhere.” When left uncorrected, high-speed-shaft bearings and generator input-shaft bearings suffer and fail at a faster rate that when corrected. In some cases the coupling also bears the brunt of neglect and fails.
Vibration analysis detects misalignment remotely and over a progression indicated by a specific vibration signature detailing the fault. Misalignment saps production and performance. The graphs in Before and after balancing shows a vibration reading or FFT spectrum between a misaligned gearbox and generator coupling. The vibration signals come from remote monitoring. The lower signal is partly due to aligned shafts.
An alignment using lasers has largely replaced older, more time consuming methods. The alignment tool mounts across the coupling with two lasers pointing at laser detectors. The shaft is rotated 180° or less and three points are taken to define a circle. The lasers represent the shaft centerlines so the difference in the two lasers provide a measure of misalignment which is corrected at least two ways:
• The generator’s mount-ing pads can be shimmed up or down to correct mis-alignment in those directions.
• The generator’s “jack bolts” can be moved side to side to correct misalignment in those directions.
If you have ever been in a wind turbine, you have noticed things move around up there. Because of the nacelle’s yawing and pitching, things may come loose. When parts break, it is usually due to being under repetitive or fatigue loads they were never intended to handle. Such failures can be real photo opportunities.
Vibration analysis detects looseness as multiple frequency peaks. One can remotely isolate the source of vibration without an up-tower climb. This is a decent alternative to climbing and checking everything that could be loose.
This one is easy – tighten what’s loose to the specified torque.
The image Before and after balancing shows vibration signals from those periods. Several components are susceptible to imbalance in a typical wind turbine. These include generator fans, generator rotors, and couplings to name a few. The causes of an increase in imbalance can range from simple things such as debris build-up on blades to material degradation, and possibly damage in the field.
Blade imbalance was once a major issue on smaller and much older wind turbines, mostly kilowatt-class machines. A common solution was to add balance weights of up to a few pounds. As the industry has grown, so have blade manufacturing methods. Much improved quality control allows creating matched blade sets weighing within close tolerances.
Using vibration analysis, you can see imbalance clearly and its degree. There are a few different types of imbalance, each having a unique vibration signature. There are also a few different types of balancing dependent upon the machine and type of imbalance detected.
Correction:There is a three step procedure for balancing in a single plane. Getting started requires a vibration analyzer, tachometer (or strobe light), vibration accelerometer, and a constant running speed. It also requires eliminating any looseness and misalignment prior to attempting to balance a component. The three-step method first calls for a:
• Baseline run. This is where vibration from the imbalance is initially measured, creating a baseline.
• A trial run then attaches a weight as a trial to induce a 30-30 rule. This means you are looking for a 30% increase or reduction in vibration, or a 30% shift in phase. This also means the trial weight has had some measurable affect on the imbalance. A calculated correction weight is placed on the machine, completing the initial balance job.
• Lastly, trim runs are performed to finalize balancing when an acceptable imbalance is not yet within tolerance. Several trim runs may be needed before the balance signal is within specs. ISO standards provide acceptable balancing specs.
Remember two simple things: Detect and correct. Tools and technology can detect these correctible conditions, then choose the appropriate corrective action for each of the three conditions.
The previously mentioned faults are avoidable and detectible using basic technologies readily available to wind farm owners, O&M providers, and manufacturers. Of course this discussion is meant to be a cursory overview of the process, not a detailed description. The details should be part of critical asset management tasks. Correcting the problems described will improve a fleet’s overall reliability and wring maximum performance from it. WPE
David Clark/Director/Turningpoint Inc., El Dorado Hills, Calif./turningpointwind.com
Filed Under: News
Dan Reilly says
Wear will cause change over time. The blades ought to have internal weights that are dynamically displaced. Software could continuously monitor vibration and make adjustments. You don’t want to train guys to go out to the field to recalibrate! Be smart about it, and design an adaptive system.
Michael Melsheimer says
I read this article with great interest. However, I might add that balancing of rotor blades still is a problem of WTG’s. We believe that about 20% of the WTG’s have problems with imbalances bigger that the allowed vibration levels. It is also known that in most cases blades in operation will change their mass moments due to wear. Both aerodynamic problems and mass moments can create vibrations which can be severe. There is a procedure can be used and it may fit smaller turbines, however a faster approach with one pass or two pass runs are available.